Temperature compensated voltage tunable circuits using surface wave devices

ABSTRACT

Electrical circuits which are sensitive to frequency drift with temperature changes are stabilized by employing a matched pair of surface wave devices; the first having a negative temperature coefficient Alpha 1; the second having a positive temperature coefficient Alpha 2. An oscillator circuit is temperature compensated by inserting a matching pair of surface wave delay elements in the feedback loop and choosing the delay time so that Alpha 1t1 - Alpha 2t2, where t1, t2 are the delay times provided by the delay elements having temperature coefficients Alpha 1, Alpha 2. A first oscillator circuit comprises a single amplifier and a pair of matched surface wave delay elements coupled in tandem in the feedback loop. A second oscillator circuit comprises a pair of amplifiers and a pair of matched surface wave delay elements alternately coupled in a closed loop configuration so that the delay elements are interlaced with the amplifiers. In a receiver system having a mixer, a local oscillator, and an IF filter for converting RF input signals to IF output signals, temperature compensation is achieved by employing a surface wave delay element in the local oscillator feedback loop and a surface wave integratable filter as the IF filter and choosing the temperature coefficients and local and intermediate frequencies so that Alpha LOfLO - Alpha IFfIF, in systems where fLO&lt;fRF and Alpha LOfLO Alpha IFfIF when fLO&gt;fRF, where Alpha LO&#39;&#39; Alpha IF are the temperature coefficients of the delay element and filter, respectively, and fRF, fIF and fLO are the RF intermediate and local oscillator frequencies, respectively. The oscillator circuits are voltage tuned by providing an intermediate electrode on the active surface of a surface wave delay element and impressing a DC signal or an AC signal or a combination of both between the intermediate electrode and the ground plane of the delay element.

United States Patent [19] Armstrong TEMPERATURE COMPENSATED VOLTAGETUNABLE CIRCUITS USING SURFACE WAVE DEVICES [75] Inventor: Donald B.Armstrong, Belmont,

Calif.

[73] Assignee: Crystal Technology, Inc., Mountain View, Calif.

[22] Filed: Jan. 28, 1974 21 Appl. No; 437,275

[52] 11.8. C1. 325/438; 325/442; 325/489; 331/107 A; 333/30 R [51] Int.Cl. H04B 1/26 [58] Field of Search 325/434, 445, 430, 438, 325/442, 489;331/96, 107 R, 107 A, 154,

72, 82 BT, 83 T, 95 S; 178/58 AF Primary ExaminerRobert L. GriffinAssistant ExaminerJin F. Ng Attorney, Agent, or FirmTownsend andTownsend [57] ABSTRACT Electrical circuits which are sensitive tofrequency drift with temperature changes are stabilized by employing amatched pair of surface wave devices; the first having a negativetemperature coefficient (1 the second having a positive temperaturecoefficient a An oscillator circuit is temperature compensated byinserting a matching pair of surface wave delay elementsin the feedbackloop and choosing the delay time so that cat, -a t where 2,, t are thedelay times provided by the delay elements having temperaturecoefficients 01 ,01 A first oscillator circuit comprises a singleamplifier and a pair of matched surface wave delay elements coupled intandem in the feedback loop. A second oscillator circuit comprises apair of amplifiers and a pair of matched surface wave delay elementsalternately coupled in a closed loop configuration so that the delayelements are interlacedwith the amplifiers. In a receiver system havinga mixer, a local oscillator, and an IF filter for converting RF inputsignals to IE output signals, temperature compensation is achieved byemploying a surface wave delay element in the local oscillator feedbackloop and a surface wave integratable filter as the IF filter andchoosing the temperature coefficients and local and intermediatefrequencies so that ca a f, in Systems Where f 0 f and a af o Olnqf whenf f where a 'a are the temperature coefficients of the delay element andfilter, respectively, and f f and f are the RF intermediate and localoscillator frequencies, respectively. The oscillator circuits arevoltage tuned by providing an intermediate electrode on the activesurface of a surface wave delay element and impressing a DC signal or anAC signal or a combination of both between the intermediate electrodeand the ground plane of the delay element.

5 Claims, 3 Drawing Figures 41 40 7 a e j meter esviee eawet.

PNEHTEHJUL 8 1975 V 14 P10 5AWDL.

MODULIATOR f TRIMMER 4 E 41 f2 40 RF INPUT MXER swF osvacs @4 5MODULATOR/ 7 TRMMER I F OUTPUT TEMPERATURE COMPENSATED VOLTAGE TUNABLECIRCUITS USING SURFACE WAVE DEVICES BACKGROUND OF THE INVENTION Thisinvention relates to electrical circuits which depend for their properoperation on the frequency stability of signals within the circuit. Moreparticularly, this invention relates to frequency sensitive circuitshaving compensating circuitry for maintaining the frequency stabilityover a range of temperatures and changes in component values.

Many electrical circuits have been developed which depend for theiroperation on the frequency stability of signals therewithin. Forexample, an amplifier having gain G (to) can be made to oscillate atfrequency w by feeding back a portion B (w) of the output signal to theinput of the amplifier and maintaining the conditions:

D B (m) w,

where t is the delay provided by the delay line. If 1 G (w) is slowlyvarying in w compared to (0,, the oscillation frequency is given by:

where n is an integer. Thus, a set of frequencies exists at which thecircuit will oscillate, provided that the above-noted amplitude criteriaof equation l are satisfied.

Since the operational characteristics of electrical circuit componentsvary with temperature and with continued use, compensation circuitry istypically provided in frequency sensitive circuits for compensating forthese changes in such a manner as to maintain the circuit operative,either automatically or in response to manual adjustment. Suchcompensation circuitry, however, suffers from several disadvantages. Onesuch disadvantage results from the fact that the additional circuitelements required to construct the compensation circuitry increase thetotal cost of fabricating the desired unit. Perhaps more seriously,however, the compensating circuitry itself is composed of electricalcircuit elements which are subject to parameter changes with temperatureand use-lifetime. As a result, the operating characteristics of thecompensating circuitry itself can change so that the original purposetherefor is defeated. Efforts to overcome the above and otherdisadvantages have not met with wide success to date.

SUMMARY OF THE INVENTION The invention comprises a method and apparatusfor providing temperature compensated, voltage tunable frequencysensitive circuits which are inexpensive to fabricate, rugged inconstruction and extremely stable in operation. In a first embodiment, apair of complimentary surface wave delay elements are provided in thefeedback loop of an oscillator circuit: the one delay element having anegative temperature coefficient a the other having a positivetemperature coefficient a which are defined for present use by theequation 01 1/: 8 t/S T. The delay elements are constructed so that 0ql=0 t where t t are the delay period provided by the first and seconddelay elements, respectively.

In an alternate embodiment, the oscillator circuit comprises a pair ofamplifiers and a pair of complementary surface wave delay elementsalternately coupled in a closed loop so that the delay elements areinterlaced with the amplifiers, with the temperature coefficient anddelay interval parameters selected as per the first embodiment.

In still another embodiment, a converter circuit including a mixerhaving an input adapted to be coupled to a source of RF signals isprovided with a surface wave integratable filter fabricated on amaterial which has a positive temperature coefficient a and a localoscillator including a surface wave delay element having either anegative or positive temperature coefficient a in the feedback loopthereof depending on whether f is less than or greater than f where fand f,;,- are the local oscillator and RF frequencies respectively. Theparameters of the circuit are selected so that a f I a{f-' in the tworespective cases where f,,- is the intermediate frequency.

The above embodiments are provided with a control circuit for enablingvoltage tuning thereof. One of the surface wave devices is provided witha control electrode intermediate the input and output transducersthereof. A suitable DC supply or AC source or a combination of both arecoupled between the control electrode and the ground plane of theacoustic wave device. By varying the DC supply or AC source, trimming,modulation, or a combination of both may be achieved.

For a fuller understanding of the nature and advantages of theinvention, reference should be had to the following detailed descriptiontaken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a partly schematicperspective view of a first embodiment of the invention;

FIG. 2 is a schematic diagram of a second embodiment of the invention;and

FIG. 3 is a schematic diagram of a third embodiment of the invention.

PREFERRED EMBODIMENT OF THE INVENTION Turning now to the drawings, FIG.1 illustrates a first embodiment of the invention comprising atemperature compensated, voltage tunable .oscillator circuit. Theoscillator circuit of FIG. 1 comprises a conventional amplifier l0 and apair of surface acoustic wave delay elements 12, 14 coupled in a closedloop configuration, with the delay elements l2, 14 coupled in tandem.

Delay element 12 is a surface acoustic wave delay line, hereinafterdesignated SAWDL, having an input transducer I5 and an output transducer16. Transducers I5, 16 are conventional interleaved combs of mutuallyspaced conductive electrodes deposited on propagation surface 17 of apiezo-electric substrate 18. The mutual spacing of the transducerelectrodes is selected in accordance with conventional techniquesto'provide Optimum surface wave generation by input transducers andoptimum transduction of the surface wave by output transducers l6.

Delay element 14 comprises a SAWDL similar to SAWDL 12 having an inputtransducer and an output transducer 21 arranged on the propagationsurface 22 of a'piezoelectric substrate 23. In addition, SAWDL 14 isprovided with a control electrode on propagation surface 22 for apurpose described below.

Temperature compensation for the oscillator circuit of FIG. 'l isprovided in the following manner. SAWDL 12is constructed from apiezoelectric substrate 18 having' a negative temperature coefficient01,, while SAWDL l4 is constructed from a piezoelectric substrate 23which has a positive temperature coefficient (1' In addition,piezoelectric substrate 23 of SAWDL 14 preferably comprises a materialhaving a highcoupling coefficient. The delay periods provided by S AWDLs12, 14' are selected so that oqt ="a t In addition, the total delayperiod T provided by SAWDLs 12, 14 is chosen so that equation 4 (seesupra) is also satisfied, i.e., I

T t -H 2mr- PG The delays provided by SAWDLs l4, 12 are simply:

where 11, 1 are the acoustic lengths of SAWDLs 12, 14 respectively, andvv are the propagation velocities of the surface acoustic wave alongSAWDLs I2, 14, respectively. Thus, temperature compensation will beachieved ifal/a2 l2/ll v /v Accordingly, given the value of thetemperature coefficients and the propagation velocities, the acousticlengths of SAWDLs 12, 14 are preselected in order to satisfy the aboutrelation.

In order to provide voltage tunability for the oscillator circuit ofFIG. 1, SAWDL 14 is provided with control electrode 25. Controlelectrode 25 is a conductive substance which is deposited intermediatetransducers 20, 21 so that an electric field may be impressed across thenarrow dimension of substrate 23 between control electrode 25 and theground plane represented by broken line '27. Control element 25 andground plane 27 are coupled via conductors 28, 29 to the output of amodulator/trimmer 30. Modulator/trimmer 30 comprises an AC source 31 andAC coupling device 32, and a DC source 33 and an adjustable resistance34 for varying the DC bias supplied to conductors 28, 29.

In use, the delay interval provided'by SAWDLs 12, l4may be adjusted byvarying delay time t, of SAWDL 14. Since delay time t is a function ofthe voltage impressed'between control electrode 25 and ground plane 27,t 'may be varied to trim the oscillator circuit frequency by merelyadjusting variable resistance 34. Likewise, the oscillator circuitfrequency may be modulated by supplying an AC bias signalfrom oscillator31 and coupling element 32 to conductors 28, 29 to impress analternating voltage of a desired frequency betw'een'control electrode 25and ground plane 27. As shown in FIG; l,this voltage tuning circuit ispreferably associated with the SAWDL 14 which is fabricated frompiezoelectric material 23 having the high coupling coefficient so thatthe largest effect can be achieved with small voltage variations. Ifdesired, however, SAWDL 12 which is fabricated from piezoelectricmaterial 18 having a low-coupling coefficient may be provided with thiscontrol arrangement.

FIG. 2 shows an alternate embodiment of the oscillator circuit ofFIG. 1. In this embodiment, a pair of amplifiers 10, 10 are interspersedin SAWDLs 12, 14 in a closed loop arrangement in order to reduce thegain requirement of the individual amplifiers 10, 10. The parameters ofSAWDLs 12, 14 are chosen in accordance with the above-noted requirementsin order to provide temperature compensation and voltage tunability.

FIG. 3 shows another'embodiment of the invention comprising atemperature compensated, voltage tunable receiver system for convertingRF input signals to IF output signals. In this embodiment, f is chosento be f A conventional RF input device 40, e.g., an antenna, is coupledto the RF input of a conventional mixer 41. The output of mixer 41 iscoupled to the input of a surface wave integratable filter 42,hereinafter designated SWIF, the output of which provides IF signals.

The remaining input to mixer 41 is obtained from a local oscillatorcomprising a conventional amplifier 43 and a single SAWDL 44. SAWDL 44is provided with a modulator/trimmer 30 of the type described above withreference to FIG. 1.

In the FIG. 3 device, SWIF 42 is a conventional surface wave deviceconstructed from a piezoelectric material which has a high-couplingcoefficient with a positive temperature coefficient a SAWDL 44 isfabricated from a piezoelectric material having a highcouplingcoefficient with a positive temperature coefficient a and is providedwith a control electrode intermediate the input and output transducersfor the purpose of providing tunability as described above. Fortemperature stabilization, the following relationoship must besatisfied: I

where f and f are the intermediate and local oscillator frequencies,respectively. It is noted that in the FIG. 3 embodiment the paramountcriterion is the requirement that the intermediate frequency match theintermediate frequency filter, rather than that the local oscillationremain stable.

The complimentary surface wave devices in the embodiments describedabove, may be fabricated from several suitable piezoelectric materials.For example, in

' the embodiment of FIGS. 1 and 2, SAWDL may be fab- X-cut quartz forpropagation directions between approximately 26 and +33 from the Y axisand Y-cut quartz for propagation between approximately i 5 of the Xaxis. Tellurium dioxide is another piezoelectric material which has anegative temperature coefficient for surface waves along severaldirections. Most piezoelectric materials have a positive temperaturecoeffi cient. The most useful, because of high coupling coefficient, areLiNbO LlTZlOg and Bismuth Germanium Oxide. As will not be apparent, theabove-described invention enables fabrication of frequency sensitivecircuits which are extremely stable over a wide temperature range andwhich are relatively simple to construct. Further. the circuits may bevoltage tuned by the provision of a simple modulator/trimmer in order tocompensate for drift in the conventional electrical circuit componentsfound in the associated elements, e.g., amplifiers 10, of FIGS. 1 and 2,and RF input device and amplifier 43 of FIG. 3.

While the above provides a full and complete disclosure of the preferredembodiments of the invention, various modifications, alternateequivalents and constructions may be employed without departing from thetrue spirit and scope of the invention. Accordingly, the abovedescription and illustrations should not be construed as limiting thescope of the invention, which is defined by the appended claims.

What is claimed is: l. A temperature compensated receiver, said receivercomprising:

input means for receiving RF signals having a freq y fm'l a mixer havinga first input coupled to said input means, and a second input, a surfacewave integratable filter having a temperature coefficient a said filterhaving an input coupled to said mixer and an output for furnishingintermediate frequency signals to an output terminal; and

a local oscillator coupled to said second input of said mixer, saidlocal oscillator comprising an amplifier and a surface acoustic wavedelay line coupled in a closed loop configuration, said surface acousticwave delay line having a temperature coefficient a said temperaturecoefficients being selected to satisfy the relation (1 f =01 f where F fare the local oscillator and intermediate frequencies, respectively,when f f said temperature coefficients being selected to satisfy therelation 1.0fm lrf II" when Luf flrF- 2. The receiver of claim 1 whereinsaid surface acoustic wave delay line includes an input transducer, anoutput transducer, and a control electrode intermediate said input andsaid output transducers; and further including means coupled to saidcontrol electrode for enabling voltage tuning of the delay time providedby said delay line to control the frequency of said local oscillator.

3. The receiver of claim 2 wherein said surface wave integratable filteris fabricated from a piezoelectric m aterial having a relativelyhigh-coupling coefficient.

4. The apparatus of claim 3, wherein said surface wave integratablefilter is fabricated from a first piezoelectric material having apositive temperature coefficient and wherein said surface acoustic wavedelay line is fabricated from a second piezoelectric material hav ing anegative temperature coefficient.

5. The apparatus of claim 4 wherein said first and second piezoelectricmaterials comprise YZ cut lithium niobate and XY cut quartz,respectively.

1. A temperature compensated receiver, said receiver comprising: inputmeans for receiving RF signals having a frequency fRF; a mixer having afirst input coupled to said input means, and a second input, a surfacewave integratable filter having a temperature coefficient Alpha IF, saidfilter having an input coupled to said mixer and an output forfurnishing intermediate frequency signals to an output terminal; and alocal oscillator coupled to said second input of said mixer, said localoscillator comprising an amplifier and a surface acoustic wave delayline coupled in a closed loop configuration, said surface acoustic wavedelay line having a temperature coefficient Alpha LO, said temperaturecoefficients being selected to satisfy the relation Alpha LOfLO AlphaIFf IF, where FLO f1F are the local oscillator and intermediatefrequencies, respectively, when f LO>f FR: said temperature coefficientsbeing selected to satisfy the relation Alpha LOfLO Alpha IFfIF whenLOf<fRF.
 2. The receiver of claim 1 wherein said surface acoustic wavedelay line includes an input transducer, an output transducer, and acontrol electrode intermediate said input and said output transducers;and further including means coupled to said control electrode forenabling voltage tuning of the delay time provided by said delay line tocontrol the frequency of said local oscillator.
 3. The receiver of claim2 wherein said surface wave integratable filter is fabricated from apiezoelectric material having a relatively high-coupling coefficient. 4.The apparatus of claim 3, wherein said surface wave integratable filteris fabricated from a first piezoelectric material having a positivetemperature coefficient and wherein said surface acoustic wave delayline is fabricated from a second piezoelectric material having anegative temperature coefficient.
 5. The apparatus of claim 4 whereinsaid first and second piezoelectric materials comprise YZ cUt lithiumniobate and XY cut quartz, respectively.